Still not fixed as of Unity 2019.1.1f1. For example, if my Renderer component (m_Renderer) has been destroyed, the following code will not work as it should:

return m_Renderer?.enabled ?? false; // throws a MissingReferenceException

Instead I have to do it like this:

if (m_Renderer != null)
{
return m_Renderer.enabled;
}
return false;

They wrote about this issue in blog post from 2014:
https://blogs.unity3d.com/2014/05/16/custom-operator-should-we-keep-it/

Ctrl-F "?? operator"

This is not fixed.
mMeshRend = mMeshRend ?? GetComponent<MeshRenderer>();
mMeshFilter = mMeshFilter ?? GetComponent<MeshFilter>();
mMesh = mMesh ?? mMeshFilter.sharedMesh;
mVerts = mMesh.vertices;

Null coalescing is only useful for cleaning up code, so it is not imperative, but it is nice to have. It should be a fairly quick to fix too.

The problem:
A) a != null ? a : b
B) a ?? b

A) Uses Object.operator==(), which is overridden for Object
B) Uses object.ReferenceEquals()

The reason why it's different is because overridden operator considers "empty" objects and "null" equals. I guess Unity uses "empty" objects for some reason instead of plain null value.

It's very annoying and hidden caveat of Unity, I wonder how they fixed it.

UnassignedReferenceException when a Transform is not assigned to a script and I try to use a ?? b instead of a != null ? a : b, even though they should be equivalent

This is not fixed.